The materials used to form an LED largely determine the emission wavelength. One group III-V composition for a light generating active layer is AlInGaP. Such LEDs typically generate light in the red to yellow range. The active layer is sandwiched between a p-type cladding layer and an n-type cladding layer, resulting in a heterostructure. The LED layers typically contain aluminum indium gallium phosphide (AlInGaP) with varying percentages of Al and In, depending on various well known factors such as lattice matching with a GaAs growth substrate and the desired bandgap. GaAs absorbs visible light, and it is common for the GaAs substrate to be removed at the end of the growth process and replaced with a transparent GaP (0% Al and In) substrate.
GaP-based materials have a relatively high index of refraction (approximately 3.5). As such, in accordance with Snell's law, a light ray is internally reflected within the LED unless the light ray impinges on a wall of the LED within about 17 degrees of normal (the critical angle). Light reflects off the internal walls of the LED until it is emitted or absorbed. Since the LED chip is substantially rectilinear, the reflected light will repeat its angle of incidence even after multiple reflections. For each internal reflection, the light becomes attenuated. Therefore, it is desirable to extract light emitted by the active layer with a minimum of internal reflections. The efficiency of a GaP based LED, after encapsulation, is about 14%, meaning that for every seven electrons that enter the LED only one photon is emitted from the LED.
What is needed is a technique for increasing the light extraction efficiency of GaP based LEDs while not adversely affecting the forward voltage.